What defines a phase in a material science context?

In the context of material science, a phase refers to a homogeneous portion of a system that has uniform physical and chemical properties. Choosing the option that describes a phase as "one individual substance in one particular state" aligns with this definition, as it emphasizes that a phase is distinct from other phases within a system. For instance, within a mixture of ice and water, ice represents one phase, while water represents another, each having different physical states and properties.

The other options describe aspects that are related but do not capture the essence of what a phase is. A physical property of matter is broader and may refer to characteristics like density or melting point, rather than defining a phase. The composition of chemical mixtures can involve multiple phases, and therefore does not singularly define what a phase is. A characteristic observable in isolation may pertain to individual properties of a material but lacks the specificity of describing the homogeneous nature that characterizes a phase in material science. Thus, the correct choice accurately reflects the definition of a phase by focusing on the specific substance in a particular state.